1. Field of the Invention
The present invention relates to a pattern inspecting apparatus and method for inspecting patterns in numerous chips formed on substrates such as semiconductor wafers.
2. Description of the Related Art
Numerous chips formed on a semiconductor wafer include repetitive pattern areas having the same pattern formed repeatedly, such as charge storage portions of memory, and random pattern areas having other patterns formed therein. In inspecting such patterns in the chips, different inspecting methods are used for the repetitive pattern areas and random pattern areas.
That is, for the random pattern areas, a chip comparing inspection is employed to detect pattern defects by comparing corresponding patterns in a plurality of adjoining or adjacent chips. This chip comparing inspection is an inspecting method also called a die comparing inspection or random comparing inspection. The chip comparing inspection compares and inspects patterns arranged in two relatively remote locations on a semiconductor wafer, and therefore needs a relatively large error allowance.
For the repetitive pattern areas, a cell comparing inspection is employed to detect pattern defects by comparing adjoining or adjacent repetitive patterns in the same chip (see Japanese Patent Publication (Examined) No. 1994-56293). This cell comparing inspection is an inspecting method also called an array comparing inspection. The cell comparing inspection compares patterns lying close to each other, and therefore is capable of a highly accurate inspection.
Thus, in inspecting the patterns in both of the repetitive pattern areas and random pattern areas in chips formed on a semiconductor wafer, two types of pattern inspecting apparatus are used, one performing the die comparing inspection, and the other the cell comparing inspection. Alternatively, an inspecting apparatus operable in two detecting modes for the die comparing inspection and cell comparing inspection may be used to perform inspections for the repetitive pattern areas and random pattern areas by switching between the two modes.
Japanese Patent No. 2976550 discloses a pattern defect inspecting method which compares cells based on images taken of chips. Cell comparable portions and cell incomparable portions are set to the image areas. A cell comparing inspection is carried out for the cell comparable portions, while a chip comparing inspection is carried out for the cell incomparable portions.
FIG. 15 is an explanatory view illustrating an operation for detecting a defect by using the cell comparing inspection.
The cell comparing inspection compares repetitive pattern A and repetitive pattern B, and compares repetitive pattern B and repetitive pattern C. An AND operation is carried out on a result of comparison 101 between repetitive pattern A and repetitive pattern B, and a result of comparison 102 between repetitive pattern B and repetitive pattern C, thereby to detect a defect 100 inherent in repetitive pattern B.
This cell comparing inspection provides highly accurate results for minute defects formed in repetitive patterns, but cannot detect defects occurring over two or more cycles of pattern repetition.
Assume that, as shown in FIG. 16, both repetitive pattern B and repetitive pattern C are defective with a relatively large particle present on these patterns. In this case, an AND operation on a result of comparison 103 between repetitive pattern A and repetitive pattern B, and a result of comparison 104 between repetitive pattern B and repetitive pattern C would result in a finding that no defect is present in repetitive pattern B.
Thus, the cell comparing inspection will fail to detect a relatively large defect occurring over two or more cycles of pattern repetition as shown in FIG. 16.
Such a problem is a phenomenon occurring also with the pattern defect inspecting method described in U.S. Pat. No. 2,976,550 noted above. With the method described in U.S. Pat. No. 2,976,550, a relatively large defect present over two or more cycles of pattern repetition when setting cell comparable portions and cell incomparable portions will be recognized as a cell comparable portion. In time of cell comparing inspection, the relatively large defect present over two or more cycles of pattern repetition will result in a finding that no defect is present.